1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber, and in particular to a small dispersion slope, low loss dispersion compensating optical fiber. This application is based on patent application No. Hei 9-252496 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
Accompanying the development of optical amplification technology, research is being carried out on increasing the strength of transmission light in order to provide better long distance transmission at wavelength 1.55 .mu.m band by inserting erbium-doped optical fiber amplifiers at the receive end, transmit end, or an intermediate point of the optical communication system.
For example, optical communication systems using optical amplifiers, such as a very-long-distance non-repeating relay and multiple-distribution subscriber optical nets, are being fabricated and researched extensively.
A dispersion shifted optical fiber having a substantially zero chromatic dispersion at wavelength 1.55 .mu.m band is desirable as a transmission line.
However, if the dispersion at wavelength 1.55 .mu.m band is small, particularly in cases where the energy density of the transmission light within the optical fiber is large, such disadvantages as nonlinear effects arise, producing a deterioration in transmission characteristics.
Thus, as a method for suppressing nonlinear effects, a method of transmission at wavelength 1.55 .mu.m using a single-mode optical fiber having a nearly zero dispersion at the conventional wavelength 1.3 .mu.m (hereafter abbreviated as "1.3 .mu.m SMF") in conjunction with a dispersion compensating optical fiber has been proposed.
For example, because the chromatic dispersion of a 1.3 .mu.m SMF is roughly +17 ps/nm/km (positive dispersion value), a large chromatic dispersion is produced when it is used to provide optical transmission at wavelength 1.55 .mu.m.
In contrast, at wavelength 1.55 .mu.m a dispersion compensating optical fiber has a negative chromatic dispersion whose absolute value is comparatively large, and can negate chromatic dispersion produced at the comparatively short distances actually used in, for example, the conventional 1.3 .mu.m SMF over several km.
Additionally, when a dispersion compensating optical fiber is incorporated into an optical system using a conventional 1.3 .mu.m SMF, even if optical communication is carried out at wavelength 1.55 .mu.m, it is possible to reduce the amount of chromatic dispersion to almost zero in the optical communication system as a whole.
Thus, in an optical communication system of wavelength 1.55 .mu.m, the wavelength distortion due to chromatic dispersion can be suppressed.
Consequently, a dispersion compensating optical fiber with low loss and a comparatively large negative chromatic dispersion at wavelength 1.55 .mu.m is necessary.
An optical fiber having a single-peak shaped refraction index profile (herein abbreviated as "single-peak profile") has been proposed as a dispersion compensating optical fiber.
FIG. 7 shows an example of a single-peak profile. The core 11 is disposed in the center, and a cladding 12 of lower refractive index than this core 11 surrounds it.
The above-mentioned core 11 consists of silica glass doped with germanium to increase the refractive index. The form of germanium used as the dopant is germanium dioxide (GeO.sub.2).
The cladding 12 consists of silica glass.
.DELTA. is a relative refractive index difference of the core 11 to the cladding 12, with the refractive index of the cladding 12 being the standard (zero).
This kind of dispersion compensating optical fiber having a single-peak profile is designed so that .DELTA. is comparatively large and has a negative chromatic dispersion, which is able to compensate the chromatic dispersion of a 1.3 .mu.m SMF. However, the problem of a large dispersion slope then arises.
When the dispersion slope of the dispersion compensating optical fiber becomes large, the dispersion slope of the system as a whole becomes large. As a result, the chromatic dispersion values of the wavelengths of the transmitted light pulse come to differ greatly. This is a disadvantage is applications such as WDM transmission which transmits a plurality of pulses of differing frequency.
Because of this, a dispersion compensating optical fiber having as small a dispersion slope as possible is needed.
On the one hand, the dispersion slope of a 1.3 .mu.m SMF at wavelength 1.55 .mu.m band is about +0.07 ps/nm.sup.2 /km (positive value). Therefore, it is further desired that when the dispersion slope of the dispersion compensating optical fiber is a negative value, it is able to compensate the dispersion slope of a 1.3 .mu.m SMF.
Thus, a dispersion compensating optical fiber that can make the dispersion slope smaller than that of a single-peak profile is desired.
To resolve this problem, a dispersion compensating optical fiber having a W-shaped refractive index profile (hereafter called a "W-shaped profile") shown in FIG. 1 has been developed and evaluated recently.
The W-shaped profile comprises a central core 21a, a middle part 21b surrounding the central core 21a and having a lower refractive index than the central core 21a, and a cladding 22 surrounding this middle part 21 and having a higher refractive index than the middle part 21 and a lower refractive index than the central core 21a.
The above-mentioned central core 21a has a bell-shaped refractive index that decreases radially. The above-mentioned middle part 21 and the cladding 22 both are annular.
Generally, the central core 21a consists of silica glass doped with germanium to increase the refractive index, the middle part 21b consists of silica glass doped with fluorine to decrease the refractive index, and the cladding 22 consists of pure silica glass.
Additionally, a is the outer diameter of the central core 21a; b is the outer diameter of the middle part 21b; .DELTA.- is the relative refractive index difference of the cladding 22 to the middle part 21 and .DELTA.+ is the relative refractive index difference of cladding 22 to the central core 21a.
.DELTA.- and .DELTA.+ show a refractive index of the cladding 22 being the standard (zero), with .DELTA.- being a negative value, and .DELTA.+ being a positive value.
The dispersion compensating optical fiber having this W-shaped profile can make the dispersion slope small, but because of the influence of the fluorine doped into the middle part 21b, there is a problem in that the transmission loss increases.
FIG. 2 is a graph showing the relation between the change in .DELTA.- as a function of the fluorine doping, and the transmission loss. The abscissa of the graph shows the absolute value of .DELTA.-. That is, the larger the value of the abscissa, the more the value of .DELTA.- shifts towards the negative, and in practice, the value of .DELTA.- becomes small.
It is apparent from this graph that when .DELTA.- becomes less than -0.2%, the transmission loss becomes extremely large.
In the conventional W-shaped profile, because .DELTA.- is designed to fall between about -0.3 .about.-0.45% in order to make the dispersion slope small, the transmission loss becomes large.
In the production of dispersion compensating optical fibers having this W-shaped profile, we believe it is advantageous to use the VAD method (Vapor-phase Axial Deposition Method) disclosed in Japanese Patent Application, First Publication Hei 7-157328 proposed by the present applicants, and to utilize a method of production by continuous manufacture of the part comprising a preform (fiber base material) for the central core 21a and the middle part 21b.
However, this method cannot under the present circumstances be utilized for a dispersion compensating optical fiber having the conventional W-shaped profile because the amount of the fluorine doping is large.
It is known that the melting point of silica glass with a dopant such as germanium or fluorine is lower than that of pure silica glass.
The method exploits this difference in melting points to produce the preform, and specifically, the following fabrication method can be conceived.
In the following preform, there are the central core base material that becomes the central core 21a, the middle part base material that becomes the middle part 21 and the cladding base material that becomes the cladding 22.
Specifically, by the VAD method, a cylindrical porous body is formed by depositing particles of germanium doped silica glass in the central part, and peripherally depositing pure silica glass particles.
When this porous body is heated in a furnace to a temperature higher than the melting point of the above-mentioned germanium-doped silica glass and lower than the melting point of pure silica glass, only the germanium-doped silica glass particles in the center is begins to progress to a transparent glass.
Further, when this porous body is heated to a temperature higher than the melting point of pure silica glass in a gas environment including fluorine, peripheral pure silica particles progress to a transparent glass, and a transparent glass rod selectively doped with fluorine in the periphery is obtained.
At this point, the germanium-doped silica glass is made into a transparent glass by the previous process, and because its bulk density is increased, it is possible to selectively dope only the peripheral pure silica glass particles with fluorine.
Furthermore, on the periphery of this transparent glass rod, by the OVD process (Outside Deposition Process), pure silica glass particles are deposited, and when heated above the melting point of pure silica glass, it is made into a transparent glass, forming the part that will become the cladding 22, yielding the preform.
Finally, this preform is drawn out forming an optical fiber having an outer diameter of about 125 .mu.m.
In continuous manufacture, this method is effective in allowing the formation of a central core base material and middle part base material having different dopants.
However, in a conventional W-shaped profile, the fluorine doped into the middle part 21b is greater than 1.2 wt. %, and because this is a large amount, the refractive index of the central core base material is lowered due to the fluorine, resulting in this method to having problems in practical application.
Anticipating the influence of this fluorine, one can conceive of a method, for example, in which the amount of the germanium doping agent is great, but then the problem of increased cost arises. Also, trial-and-error preliminary experiments to ascertain the materials formation and production conditions are necessary.
For this reason, dispersion compensating optical fibers having a W-shaped profile are manufactured by a method wherein the central core base material that becomes the central core 21a is produced by the VAD method, and then the fluorine-doped silica glass that becomes the middle part 21b is added to the outside.
However, this method caused a roughness on the outer peripheral surface of the central core base material due to the large amount of germanium doping. Consequently, an operation which smoothes this surface by external abrasion is necessary.
An eccentricity in the central core base material is produced due to this external abrasion, a new problem arises in that the polarization mode dispersion becomes large.
In addition, the production processes are complicated, and there is the possibility of contamination of the surface of the central core base material during production.